Ion sources are employed with mass spectrometers in the analysis of substances. Commonly used sources are the electron impact source and the chemical ionization source. The first one has a large electron entrance, a large ion exit, and an ionization region where the incoming electrons fragment as well as ionize vapor molecules thus providing a large quantity of information which does not necessarily give clear indication of the identity of a substance. The chemical ionization source has, on the other hand, a small electron entrance, a small ion exit, and an ionization region where the pressure can be maintained at such levels that ion-molecule collisions are extremely likely to occur, such collisions leading to ready identification of the molecular weight of a substance.
Operation of a mass spectrometer alternately with electron impact and chemical ionization sources has required many hours of down time during which the operation of the spectrometer stops. An object of this invention is to permit changing between the electron impact and the chemical ionization configurations with minimal interruption of operation of the mass spectrometer.